[Bug #21448] Use `getentropy(2)` only on macOS

	If this is not a system call, then it is using getrandom (which would
	have been tried already), and cannot be used as a replacement for the
	random devices.

	[Bug #21448] Reorder trials in `fill_random_bytes`

	First try dedicated system calls, such as `getrandom` or `getentropy`,
	next possible libraries, then fallback to `/dev/urandom`.

`len` is always at least 1 here.

When loading a crafted marshal data of Random, a pointer to an illegal
address was created. I don't think there is any harm since the data is
normalized before access, but just to be safe, I add a check to make it
an error.

Do not retry `fill_random_bytes` in `Init_RandomSeedCore`, just after
it failed first.

In other places, `rand_init_default` and `random_seed`, no more tweeks
needed once it succeeded

Also use a typed data.

This `st_table` is used to both mark and pin classes
defined from the C API. But `vm->mark_object_ary` already
does both much more efficiently.

Currently a Ruby process starts with 252 rooted classes,
which uses `7224B` in an `st_table` or `2016B` in an `RArray`.

So a baseline of 5kB saved, but since `mark_object_ary` is
preallocated with `1024` slots but only use `405` of them,
it's a net `7kB` save.

`vm->mark_object_ary` is also being refactored.

Prior to this changes, `mark_object_ary` was a regular `RArray`, but
since this allows for references to be moved, it was marked a second
time from `rb_vm_mark()` to pin these objects.

This has the detrimental effect of marking these references on every
minors even though it's a mostly append only list.

But using a custom TypedData we can save from having to mark
all the references on minor GC runs.

Addtionally, immediate values are now ignored and not appended
to `vm->mark_object_ary` as it's just wasted space.

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>

* Make its usage more obvious (by providing more detailed
  examples);
* Include mention of it into Random itself;
* Small cleanups of docs.

Distinguish initialization by single word from initialization by
array.